Shooter Tolerances, Part 3: Wrapping Up

I promised to wrap up this topic. Here goes. Along the same lines as last time, the target is still 4”. The following examples assume a perfect zero, a 168 grain .308 Hornady A-Max with a muzzle velocity of 2650, and a sight height of 1.6” (basically a typical .308 bolt action rifle).

I think I pretty much covered why thinking of the rifle’s point of impact as a point was not a good idea. A zone would be more appropriate. Allow me to beat a dead horse.

Perfectly zeroed shot:

Perfect Zero, no wind

It would take a 28 MPH full value wind (3 O’clock) to push the round to the edge of the target.

Perfect Zero 28 mph wind

28 MPH sounds like a ton of leeway. A real-deal, no-foolin’ 10 round, 1 MOA group at 100 will be right at the edge with a 20 mile per hour wind. But my rifle will rarely shoot a 10 shot MOA group. There are also times when prone with the bipod and rear bag aren’t feasible for whatever reason.

You may remember that in installment 1 of this series I stated the following:

-Exertion decreases precision and accuracy.
-Time stress decreases precision and accuracy.
-Precision and accuracy tend to degrade proportionally with the height of the shooting position from
the ground.

Along those lines, I found that in the last few months, the only example that I found of a seated group that I shot under time stress and exertion consisted of only 5 shots and was approximately 3.3 MOA. The following group is not identical, but should do for the sake of discussion:

Perfect zero, 3point3 MOA exerted sitting, no wind

The shooter tolerances being increased in this example means that a 3 O’clock wind of only 4 MPH will now conceivably put me at the edge of the target.

Perfect zero, 3point3 MOA exerted sitting, 4 mph wind

I could go on and on with more examples, but I think there’s enough already to make the point I want to make. I should also say that by using groups as my illustration, the extreme spread tends to stand out, and the extreme spread is not the most statistically significant measure of performance since the outliers tend to be most prominent. The last group with five rounds is also not a large enough sample size, but since I actually shot a group that large I thought it somewhat relevant.

Here’s the point of all this. I told you I was going to explain why I spent so much time working on getting my zero just right. All of the examples from the previous articles assumed a perfect zero. I don’t know if I’ve ever had a perfect zero. The one I posted might be the best I’ve had.

Cumulative Group
If I tried to get a better zero I’d overshoot and be worse off.

If the other articles demonstrated how easily our ballistics charts can essentially lie to us because of the phenomenon of grouping, if the safety and surety of the point blank zero can be undone with a bit more wobble, if a little wind can become a big problem with realistic dispersion, what happens to all that when the rifle isn’t actually zeroed?

If I do what I normally have done, and after zeroing with 3 or 5 rounds, put it in my mind that “I’m zeroed” and move on to the next thing, it might be frustrating when I start seeing rounds several inches off target, and there’s “barely” any wind, I “know” I’m zeroed, I’ve shot a few sub-minute 5 shot groups with the rifle, etc… It takes some time, patience, and more than a few rounds downrange to get a realistic idea of the system’s capabilities alone. I would recommend stacking several 10 round groups on top of each other with the same sight setting before considering the system zeroed. Anything that compromises the shooter’s ability to maintain a steady hold gets added on top of that, and that takes a long time to learn as well. Precise measuring, good documentation, and use of analyses that are more statistically significant than the old standby, extreme spread, will all help.  I should also note that it’s possible with a lot of the ballistics programs to input the variance of the loads zero from the point of aim at the zero range, which will allow for a better use of that technology.

Good luck.

7 thoughts on “Shooter Tolerances, Part 3: Wrapping Up

  1. Ah reality invades gun magazine baloney-
    reality : time ,ammunition ,opportunity = zero +
    external factors, health ,breathing, environmental conditions = POA
    rough formula
    It did not go unoticed that you signed off “good luck” that is the x factor
    BTW do you know the range at which the Spokane deputy make the hand shot on women brandishing the pistol the other day?

  2. Tap, Tap,
    is this thing on?
    Rifleslinger, are you going to continue posting?
    Or, have you just been too busy to post?

    Rick

    • Yessir. I have just been running beyond capacity, burning the candle at both ends, running around like a chicken with my head cut off… you get the idea. I hope to be back posting soon and making slings as well.

  3. Have you taken a look at Brian Litz’s Weapons Employment Zone (WEZ) analysis……..He covers this topic very well.

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